Another update. I have found a superb forum here:
Clogged Canon print head | PrinterKnowledge It was started five years ago and is still going strong. The amount of DIY expertise and information with regard to cleaning and maintenance of inkjet heads on this forum is quite staggering. I was curious to discover whether it was possible to strip down a printer head to see where the problem might be. The answer is yes, provided you have some DIY or engineering knowledge or both. However, there is a lot of advice there regarding removing blockages before getting to the strip-down stage. I am minded to buy the MG3250 anyway, change the head over and then work on the old head. Watch this space.